The invention relates to a method for reducing the number of leukocytes in blood through treating the blood in an arrangement of a plurality of fibers based on organic polymers, whereby the blood is channeled along the fibers and whereby the reduction in leukocytes from the blood occurs essentially through adsorption on the fibers, characterized in that the plurality of fibers based on organic polymers is fixed in a housing with an inlet arrangement and an outlet arrangement, and that the blood is channeled via the inlet arrangement into the housing, flows through the housing with the plurality of fibers made from organic polymers, and exits the housing via the outlet arrangement, that the arrangement of the fibers shows a high degree of order, that the blood flows along the external surface of the fibers, and that the fibers cause a generation of the complement activation product C5a in a concentration of at least 10 μg per m2 of fiber surface.